1. Field of the Invention
This invention relates to a unit type fuel injector for fuel injection in an internal combustion engine.
2. Description of the Prior Art
The conventional unit type fuel injector has a fuel injection section for injecting pressurized fuel received from a fuel pressurization section for pressurizing the fuel and is constituted so that the pressure in a fuel pressure chamber of the fuel pressurization section can be spilled to a fuel supply return chamber provided in a solenoid valve section by lifting a valve needle in the solenoid valve section off the associated valve seat cone. The fuel injection timing can be controlled by controlling the timing at which the needle is seated on the seat cone. However, since the pressure in the fuel pressure chamber of a fuel injector of this type is extremely high, the pressure in the fuel supply return chamber rises rapidly at the time the needle of the solenoid valve section is lifted off the seat cone for spilling the pressure in the fuel pressure chamber. Since the pressure this produces in the fuel supply return chamber causes a force to act on the needle in the opposite direction from that for lifting the needle off the seat cone, the pressure in the fuel pressure chamber cannot drop sharply. This degrades fuel injection cutoff.
For overcoming this problem, Japanese Patent Application Public Disclosure No. Hei 2-67455(67455/1990) teaches a fuel injector in which the valve needle is formed on one end of a needle valve rod, a flange is provided on the needle valve rod to be in liquid-tight sliding contact with the wall defining the space surrounding it, and the pressure produced in the fuel supply return chamber by the separation of the needle from the seat cone is caused to act on the flange in the direction causing the valve to open. More specifically, the proposed fuel injector is configured so that the pressure increase produced in the fuel supply return chamber at the time the needle rises off the seat cone is led through a passage in the needle valve rod to the opposite side from the needle and applied to the flange. This configuration is disadvantageous in that the force produced by the rapid pressure increase arising in the fuel supply return chamber at the time the needle lifts off the seat cone acts directly on the needle in the direction causing it to move toward the seat cone before the pressure led to the flange can act on it in the opposite direction. Valve opening therefore tends to be hindered even when the control current to the solenoid in the solenoid valve section is turned off.
If in order to overcome this disadvantage the backward speed of the valve body is increased to quickly lift the valve body of the solenoid valve off the seat cone, the valve body will collide strongly against the stopper in its backward operation, causing the valve body to bounce. Furthermore, this will disadvantageously cause secondary injection to occur.